Alertly

Behind one of the old oaks which we have described in the preceding chapter, shrouding himself from observation like a hunter watching for his game, or an Indian for his enemy, but with different, very different purpose, Tyrrel lay on his breast near the Buck-stane, his eye on the horse-road which winded down the valley, and his ear alertly awake to every sound which mingled with the passing breeze, or with the ripple of the brook.

Source: wiktionary

I walked fast to avoid being overtaken. Every crossing was a danger, every passenger a thing to watch alertly.

Source: wiktionary

Miss Baker and I exchanged a short glance consciously devoid of meaning. I was about to speak when she sat up alertly and said “Sh!” in a warning voice.

Source: wiktionary

She was wearing tight white shorts and a grey tank top of Jasper’s, in which her small breasts moved alertly.

Source: wiktionary